Snapchat used to sell additional Replays, but no longer offers them for sale. If you purchased additional Replays in the past, you can still use them, but you can no longer purchase them. [2] X Research source

If you have multiple unseen Snaps from that person, they will all play back-to-back, and using a Replay will play through all of them again.

When you do this for the first time after updating, a window will appear asking you to confirm that you want to replay the Snap. If you want to use a Replay for a specific Snap in a group of Snaps you received, swipe right on the Snap to open a chat with the person. Press and hold on the Snap that you want to replay in the chat history. You can only replay Snaps that you just received. Exiting the chat or the inbox screen will remove your ability to replay the Snaps.